1. Field of the Invention
The invention relates to an electromagnetic deflection unit for a colour display tube comprising a conical coil holder which carries a set of coils for the horizontal deflection of electron beams, which coils have segments which extend substantially in the longitudinal direction of the coil holder, soft-magnetic members being arranged on the inwardly facing side of said coils in the proximity of the said segments. The invention further relates to a colour display tube provided with such a deflection unit.
Such a deflection unit and a colour display tube provided with such a unit are commercially available.
2. Description of the Prior Art
Current techniques of winding deflection coils are not sufficiently good to manufacture coils having such a distribution of turns that all the requirements with respect to convergence and raster are satisfied simultaneously. The known unit comprises a plurality of soft-magnetic members which influence the magnetic field of the set of coils producing the horizontal deflection and thus reduce residual convergence errors in the corners of the display screen. In contrast therewith, geometrical (raster) errors are corrected, if required, by means of permanent magnets which are mounted outside the coils on the side of the coil holder to be directed towards the display screen.
The number, the shape and the size of the members as well as their position with respect to the set of coils for horizontal deflection have to satisfy stringent requirements in any type of deflection unit. However, these requirements are the same for units of a given type. The soft-magnetic members therefore are not intended to be used for neutralizing any differences in the properties of the units of one type, but for correcting the residual errors with regard to the convergence in the design of a type of deflection unit.
The step of arranging and fixing the members, which may be of different shape and size, in the correct position is time-consuming and hence expensive, the more so as a comparatively large number, for example, four, six or eight members may be concerned. In order to be able to produce a maximum field strength with a minimum of power, the coils for horizontal deflection are generally located within the coil holder so that--when the deflection unit is mounted on a display tube--the coils are arranged as close as possible to the display tube. This construction of a deflection unit results in that the members generally have to be adhered for the major part to the inner surface of the coils. The fairly uneven surface of the coils can, however, lead to a poor adherence, as a result of which members become detached. It is usual to position the soft-magnetic members by means of adhesive tape and to fix them by means of an adhesive.